1. Field of the Invention
This invention relates to a process of producing a polyimide asymmetric membrane having a dense layer and a porous layer. More particularly, it relates to a process of producing a polyimide asymmetric membrane made of multi-component polyimide containing a fluorine-containing polyimide and having a controlled composition of the fluorine-containing polyimide in its dense layer. The process of the invention provides a practical high-performance gas separation membrane.
2. Description of the Related Art
Gas separation membranes are made use of in industrial gas separation processing. Above all, gas separation membranes made of polyimide with high permselectivity (permeance ratio) are used. Generally speaking, polyimide has high permselectivity (permeance ratio) but low permeability (permeability coefficient). Hence, a gas separation membrane made of polyimide has an asymmetric structure composed of a porous layer primarily performing a mechanical supporting function and a dense layer primarily performing a separation function, with the thickness of the dense layer, where permeate gas undergoes permeation resistance, reduced so as to secure a gas permeation rate.
A gas separation membrane for practical use is required to have not only gas permeation characteristics including gas permselectivity and permeation rate but other characteristics such as mechanical strength. In the case of polyimide derived solely from one tetracarboxylic acid component and one diamine component (i.e., homopolyimide), these characteristics are determined by the combination of the tetracarboxylic acid component and the diamine component. In order to realize a gas separation membrane fulfilling these requirements for practical use, studies of gas separation membranes have been directed to use of copolyimide obtained by replacing part of the tetracarboxylic acid component and/or the diamine component with other tetracarboxylic acid component and/or other diamine component. Characteristics of gas separation membranes made of such copolyimide depend on the composition of two or more tetracarboxylic acid components and/or two or more diamine components. Through the studies, polyimides prepared using a fluorine-containing tetracarboxylic acid component or a fluorine-containing diamine component have often been used for the purpose of improving gas permeation characteristics, particularly permeation rate.
In general, nevertheless, an asymmetric membrane formed of a polyimide with excellent gas permeation characteristics, such as a fluorine-containing polyimide, has insufficient mechanical strength, while an asymmetric membrane formed of a polyimide with high mechanical strength exhibits insufficient gas permeation characteristics.
JP-A-6-269650 discloses a composite gas separation membrane having a laminate structure comprising (a) a porous polyacrylonitrile structural support material, (b) a gutter layer comprising a crosslinked polar phenyl-containing-organopolysiloxane material, and (c) an ultrathin selective membrane layer comprising a specific fluorine-containing polyimide.
JP-A-8-52332 discloses a composite gas separation membrane comprising an aliphatic porous polyimide supporting layer and a fluorine-containing polyimide thin layer laminated thereon.
Making such a composite membrane involves forming a uniform thin layer on a porous layer. However, it is not easy to uniformly form a thin layer on a porous layer. In fact, it is not easy even with the processes taught in the above references to obtain a high performance gas separation membrane.
Japanese Patent Application No. 2003-24755 discloses a process of producing an asymmetric hollow fiber separation membrane by phase inversion method using a polymer blend solution containing two kinds of polyimides. The reference does not mention production of an asymmetric membrane using a blend solution containing a copolymer having “blockness” that is obtained by preparing a blend solution containing polyimide components having specific polymerization indexes and further subjecting the blend solution to polymerization and imidation.